1. Field of the Invention
The present invention relates to systems for minimizing interference caused by mobile radio stations initiating and terminating communication with fixed radio stations.
2. Description of Related Art
In cellular radio telephone networks, a mobile subscriber may freely choose when and where to initiate a telephone call. This procedure is known as a random access call set-up. The term random access also applies to the mobile station's first transmission in reply to a call initiated through the mobile station's fixed home base station. In both situations, significant uncertainty exists in determining the mobile's transmission power level at the time of access.
Three principal methods enable a radio telephone system to support multiple, ongoing conversations in a given frequency band. Frequency Division Multiple Access (FDMA) is the traditional method, where every call connection between a mobile and a base station is allocated a unique frequency channel that is occupied continuously until the end of the call. At present, mobile telephone systems are changing from FDMA to time-based methods of sharing communications resources. In Time Division Multiple Access (TDMA), different radio transmitters are allocated short time slots in a periodic cycle in which they transmit bursts of information. In the third approach, Code Division Multiple Access (CDMA), different speech/information signals are transmitted with different spread-spectrum codes so that the coded signals overlap in both time and frequency. The received CDMA signals are decoded by correlation with the code associated with the desired speech/information signal.
In all mobile telephone systems, the physical distance between mobile stations and base stations varies significantly. The signal propagation loss between a radio transmitter and receiver varies as a function of the fourth power of their mutual distance. As a result, large differences may arise in the strength of signals received at the base station from different mobiles. Although conventional cellular radio telephone systems employ a number of techniques to avoid interference between different signals, interference occurs nonetheless as the disparity between the signal strengths from various mobiles increases.
This interference problem is of particular concern in CDMA systems where a mobile signal that is twice as strong as another mobile signal occupies twice the system capacity. Unregulated, it is not uncommon for a strong mobile station to transmit signals at thousands of times the strength of other mobile transmissions. The loss of system capacity to such "strong" mobile stations is unacceptable, and thus power regulation is particularly important in CDMA systems. In commonly assigned U.S. patent application Ser. No. 07/866,554, entitled "Duplex Power Control" filed on Apr. 10, 1992, the present inventor describes a power regulation method and apparatus for a CDMA system. That application is incorporated herein by reference.
In other pending applications by the present inventor, U.S. patent application Ser. No. 07/628,359, filed Dec. 17, 1990, and entitled "CDMA Subtractive Demodulation," now U.S. Pat. No. 5,151,919 and U.S. patent application Ser. No. 07/739,446, filed Aug. 2, 1991, and entitled "CDMA Subtractive Demodulation," now U.S. Pat. No. 5,218,619 incorporated herein by reference, a CDMA subtractive demodulation system is described in which overlapping, coded signals are decoded in the order of strongest to weakest signal strength. After each decoding, the decoded signal is removed or subtracted from the received, composite signal before decoding the next strongest signal. Using such a CDMA subtractive demodulation system, signal strength differences between mobiles become less important and capacity is increased. In other words, the signals having the greatest potential for causing interference, i.e., the strongest signals, are decoded and removed first. In this way, potential sources of interference for weaker signals are significantly reduced.
Even in such a CDMA subtractive demodulation system, however, an interference problem still exists when mobile stations initiate random access call set-ups. Because of the difficulty in gauging an appropriate access power level, there is a risk of at least momentarily interference with ongoing conversations.
Another source of potential interference to ongoing conversations during mobile random accesses is time misalignment of the mobile random access signals relative to the base station's frame timing. For mobile station signals to be received in a correct timeslot (TDMA) or correctly time-aligned to a particular correlating code (CDMA), the mobile station must adjust its access signal transmission timing to account for the round-trip propagation delay between the base and the mobile station. Unfortunately, unless a recent contact has been made with a base station, the mobile station lacks a mechanism for establishing the correct time alignment for a random access.
These problems undermine the efficient operation of current and future cellular systems. Given the frequency with which new calls are placed by mobile telephone subscribers, especially in urban and other congested areas, it is both desirable and necessary for mobiles to make random accesses on the network without generating unnecessary interference. It is also desirable to simply and effectively establish a call connection from mobile to base station that is synchronized with the time-alignment structure of the base station.